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Dear Answer Boy: What makes cancer invade certain parts of the body and not others? Is there something unique about the tissue of the breast, prostate, stomach, rectum, or liver? I guess what I need to know is, how com I never hear about cancer of the butt? —Anally Curious, Oakland

Dear Matthew: Why is the heart the only organ that does not get cancer? Dennis, San Diego

Our pal Doctor Doctor was busy digging his cart out of a sand trap on the back nine, so I ran this one past the American Cancer Society and its consultants. Start preparing your crib sheets. There will be on the exam.

To begin with, what’s cancer? A general term for about 100 separate diseases, all characterized by uncontrolled proliferation of body cells. What cells in the body are potential cancer sites? Theoretically, all of them. Can you get cancer of the butt muscles, heart muscle? Theoretically, yes. Happen often? So far, never (I’m prepared to be contradicted on this one, though). So what’s the deal?

We all know that bur body cells die and replace themselves in seven-year cycles, yes? Bzzzzz Wrong, mitosis-breath. Some, especially our protective epithelial cells, are programmed to replace themselves very fast. On the other hand, nerve and muscle cells never renew themselves. (The ones you have can get bigger, you can get new ones, and some can die, but it’s not a renewal cycle.) Which type of cell do you think is more sensitive to a cancer signal that says, “Gentlemen, start your replicators”? Likewise, if cancer starts in one spot and is carried through the blood or lymph another, it doesn’t find as friendly a home in your glutes as in your lungs. This, at least, is one credible if simplified theory for why cancer prefers certain locations.

Soft-tissue cancers are called sarcomas. Rhabdomyosarcoma is cancer of striated muscle, e.g., your butt, your heart. Cancer of smooth muscle (e.g., uterus lining), leiomyosarcoma. Cancerous fat cells? Liposarcoma. Collagen tissue (bones, tendons, cartilage, connective tissue)? Fibrosarcoma. Joint lining? Synovial cell sarcoma. Kaposi’s sarcoma is a soft-tissue tumor that can arise anywhere blood vessels exist.

Heart muscle is covered by two protective membranes collectively called the mesothelium, site of malignant mesotheliomas. Like sarcomas, they’re rare, but they’re there. Take care.

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